I am in the midst of a prolonged discussion with some friends about when to use bike lights. I always have mine on in low-light or nighttime and in bad weather, like fog or rain. If I ever feel like cars might have difficulty seeing me, the lights go on. An alternate view is for the lights to ALWAYS be on, regardless of visibility. On a bright, sunny day, having lights on seems like a waste of batteries because that light is washed out by the much stronger sunlight.
Over my past few rides, I directed a good deal of thinking towards lighting. I came up with an extremely simple rule for when to have lights on/off.
The Groundhog Rule:
If you can’t see your shadow, your rear lights should be on flashing.
Addendum: If you can’t see the road easily, rear lights should be on solid, and your front light should be flashing if day, solid if dusk or night.
I also posed this question to the Ultracycling group on Facebook to see their responses as well, since those riders typically have more experience than myself in being on the road all the time. As those responses come in, I will summarize them below.